Marine sharks except for some freshwater rays contain urea, one of the most typical protein denaturants, in the range of 0.2 to 0.6 M to exert resistancy against high osmolarity of seawater. However, proteins in urea-containing sharks can maintain their physiological functions even in the presence of urea as proteins of teleost in the absence of urea. The proteins from marine sharks are supposed to be protected from denaturation by some molecular mechanisms. The objective of this study was to analyze the primary structure of freshwater ray myosin and to compare those of requiem shark to test the above assumption on the molecular mechanism underlying urea-resistibility of marine shark myosin.The myofibrillar Mg^<2+>-ATPase activity of requiem shark was gradually decreased with increasing urea concentrations, showing maximal value at around 0.3 M urea and then declined gradually with increasing urea concentrations. On the other hand, the activity of carp was decreased by the addition of
… Moreurea even at a low concentration and declined rapidly with increasing urea concentrations. The activity of freshwater ray was nearly the average of the relative activities of requiem shark and carp. These results indicated that the myosin from freshwater ray is suitable material for the research of the urea-resistibility.The amino acid sequence of skeletal muscle myosin from freshwater ray was deduced from cDNA nucleotide sequence, predicting 1933 amino acid residues. In this study,1788 amino acid residues from freshwater ray myosin were compared with those of requiem shark and other teleost. The residues 785-793 located in essential light chain binding site in the myosin heavy chain S1 region. This amino acid sequence of requiem shark and freshwater ray myosin showed extreme hydrohilicity comparing with those of other teleost, but the residues 797-799 showed hydrophobicity. The residues 685-693 located around the ATP and actin-binding site in S1 region. This sequence of freshwater ray myosin showed the hydropathy between carp and shark. This region is close to the ATPase activity domain, and the relevance to the urea-resistibility was indicated. The obtained results elucidate a part of molecular mechanism of urea-resistibility of the marine shark. Less